ESS 7ESS 7Lecture 17Lecture 17

November November 9, 20119, 2011

TheThe AuroraAurora

Aurora – Amazing Light Show

Andrew Keen, Inari, Finland 18 Feb 2010

-31º C

spaceweather.com

Athabasca Aurora Oct 3 2003

Courtesy Mikko Syrjäsuo

• The record of auroral

observations dates back

thousands of years to

Greek and Chinese

documents.

• The name aurora borealis

There is a Long Record of Auroral Observations

• The name aurora borealis

(Latin for northern dawn)

was coined in 1621 by

either Galileo or his

colleague P. Gassendi

during a spectacular event

observed in southern

France.

What are Aurorae*?

• The aurora is mainly caused by excitation due to

precipitating electrons and ions. Auroras typically are

found at high geomagnetic latitudes where

magnetospheric and solar wind electrons can readily

access the upper atmosphere.access the upper atmosphere.

• Typically 1011Js-1 is required to maintain auroral

emissions – this is about twice the maximum generating

capacity of California. A Js-1 or joule/second is a watt

(W). This power of the aurora is thus 100,000 MW.

*Aurorae=plural; often also “auroras”

• Auroral emissions are primarily due to a two-step process in which precipitating energetic auroral particles collide with the atoms and molecules of the Earth’s upper atmosphere.

• Part of the particles kinetic energy is converted into energy stored in the chemically excited states of atmospheric species.

Auroral Light

atmospheric species.

• The excited states relax giving off photons.

.

• The brightest visible feature of the aurora, the green line at 557.7nm is due to the transition of an electron from 1S excited state to the 1D state of atomic oxygen.

Chemically Excited States• Another commonly observed line particularly in the polar

cusp and cap is the red line at 630 nm. This occurs as the 1D state relaxes to the ground state (3P2)

.

followed by

')()(13

eSOePO +→+

)7.557()()(11

nmhDOSO υ+→or

For the red doublet

followed by

(e’ has less energy than e)

• If the O(1S)-state electron gives up its full energy in a single step, instead of two, it emits a 297.2 nm photon.

)7.557()()(11

nmhDOSO υ+→

)2.297()()(31

nmhPOSO υ+→

')()(13

eDOePO +→+

)4.636/630()()(31

nmhPODO υ+→

• Perhaps the easiest way to see the transitions is by looking at a chart showing the different levels.

• The line at 557.7 nm is called a forbidden line.– Allowed transitions occur much

more rapidly (10-7s) than

Auroral Spectra Contain Lines from Many Transitions

more rapidly (10-7s) than forbidden transitions (0.8s in this case).

– Forbidden transitions occur at high altitudes (>200km) since at lower altitudes they have a good chance of being knocked out of the state before they can emit.

• There are many permitted oxygen and nitrogen lines from higher excited states.

The Most Common Visible AuroralEmissions are from Neutrals

• The 630nm emission forms the diffuse background radiation in which the discrete arcs are embedded.

• “Blood-red” auroras are produced by low-energy electrons (<<1 keV) they are found at high altitudes (>200km)

• Magenta predominates below 100 km and is a

combination of N2 and O+

2 emissions near 600nm and N+

2 violet emissions.

The Colors of the Aurora

found at high altitudes (>200km)

• “Red lower borders” indicate the presence of energetic particles (>10keV).

• Most auroras are yellow-green but sometimes appear gray (because our eyes are insufficiently sensitive)

• There are four forms of aurora.

– Quiet homogeneous arcs stretching in an east-west direction across the sky.

– Auroral rays .

The Shapes of Auroras

– Diffuse auroral clouds.

– Spirals and curls.

Diffuseaurora

Arcs

Courtesy Jan Curtis

Shapes and Colors

Rays Arcs and Rays

Curtain, Drapes

Diffuse and Discrete Aurora

• In addition to discrete auroral structures seen in the images there is a background aurora

called diffuse aurora.

• The discrete aurora can be seen as peaks in the radiance

curve at the right. curve at the right.

• The discrete aurora rise out of the diffuse aurora which extends over a much wider range of latitude.

• A typical auroral arc receives about 106 kW – a large power plant.

Note: logarithmic scale. The eye

detects on a logarithmic scale.Only in this way can we see at night

and also during the day!A camera needs many changes of setting for night/day; we don’t!

• Aureol-3 (French for aurora) observations of electrons and ions precipitating into the nightside ionosphere.– Middle panel shows average

energy

– Bottom panel shows the

The Particles that Excite the Aurora

– Bottom panel shows the energy flux to the ionosphere.

• Ion signature around 2124 UT is a velocity dispersed ion structure thought to be the signature of the outer edge of the plasma sheet (PSBL).

Particles Making Discrete and Diffuse Aurora

• Red electron fluxes equatorward are “inverted V” events associated with discrete aurora.

• Precipitating particles • Precipitating particles equatorward of 68.50

produce the diffuse aurora.

The Auroral Oval

• The aurora are found in rings about the north and south poles.

• These are magnetically connected to the connected to the equatorial magnetosphere.

• The sketches on the right are based on observations and show the regions with aurora for quiet and disturbed times.

• Disturbed auroral oval occurs for southward IMF during storms and substorms.

• When the magnetosphere is quiet and the IMF is northward polar-cap sun

Polar Cap Sun-Aligned Aurora

northward polar-cap sun aligned arcs oriented from midnight to noon can occur.

• They occur about 50% of the time when the IMF is northward.

Image of “theta aurora” during the passage of a magnetic cloud past the Earth

Assignment

• Read Chapter 6